Currently there are two tools for managing online accounts available in Ubuntu, the Ubuntu Online Accounts and the Gnome Online Accounts.

There are also two kinds of software using either of these two systems, most noticeable in case of Empathy. It can access Google Talk accounts though the Ubuntu system but also integrates with gnome-contacts. It in return use Gnome online accounts to access the same Google accounts for contact data.

Since the point of these systems is to only manage your accounts in one place it is rather unwelcome to maintain accounts in both.